Broadhaven Bay

Overview

Broadhaven Bay is located on the Atlantic coast, north Co. Mayo in the north-west of Ireland. It is open to the North Atlantic at its northern end. At its southern end it narrows at Belmullet and is linked to Blacksod Bay by a narrow man-made channel.

INFOMAR Survey History

The Celtic Voyager mapped a large portion of the deeper water in 2002. The RV Geo, RV Keary and Cosantóir Bradán focused their efforts on the shallower waters in the bay in 2010, 2011 and 2014.

A Lidar survey was completed in 2010 by Pelydryn. The seabed is predominantly sand and good laser penetration and returns were experienced throughout the Bay. Survey lines were extended outside of the Bay itself in order to give an area of overlap with existing multibeam surveys.

Shaded Relief

The 3D appearance is achieved using software called Fledermaus. By using some vertical exaggeration, artificial sun-shading (usually as if there is a light source in the nw 315°) and colouring the depths using various colour maps, it is possible to highlight the subtle relief of the seabed. This helps us to quickly understand the variation in depths.

Click image for more detailed map.

All of our Charts for Broadhaven can be downloaded in pdf format from this page.

Backscatter

Multibeam Systems also collect additional information, including the strength of the acoustic signal (or return) from the seafloor. This is known as Backscatter. Differing seafloor types, such as mud, sand, gravel and rock will have different Backscatter values depending on the amount of energy they return to the sonar head. Rocky areas will typically have high returns while soft sediments like mud are more likely to absorb energy and have low Backscatter returns. These differing values are used to generate a grey-order image (i.e. dark for high returns, bright for low returns) of the seabed which can be used to examine the nature of the seafloor.

Ground Truthing/Seabed Sampling

To verify the results of remotely sensed sonar data, it is important to collect physical sediment samples from the seabed. These may be from the surface where grabs are used or may penetrate through the seabed and retain the vertical structure of the sediment by using various coring methods. These samples are also critical for the confirming seabed classifications.

The RV Keary took 13 grab samples (red circles)in Broadhaven Bay in 2014. All the samples were sand.Full Particle Size Analysis (PSA) was carried out on the samples.

22 samples (yellow triangles) were taken in Broadhaven Bay in 2009 by the Marine Institute. Most of the samples were Sand. 5 were Slightly Gravelly Sand Full PSA was carried out on the samples. 33 samples (green triangles) were taken in Broadhaven Bay in 2012 by the Aquafact mainly using a grab sampler and a couple using a Van Veen grab smapler. The majority (19) were 'Slightly gravelly sand' and the remainder were sand (14). Full PSA was carried out on the samples.

Offshore sampling (64) was taken using a vibrocorer in 2010 by the Celtic Voyager (blue circles). Geotechnical only with no PSA carried out. The Celtic Voyager also ran a sampling campaign (red circles offshore) in 2002. 7 of the samples can be seen in the image. The 4 samples in the top right of the image with dark backscatter are Course sand and gravel.